Regarding a source of brass for the .308 Norma magnum - if you run out of or can't locate Norma brass, in the old Speer reloading manual number 8, they noted that the Norma case is close dimensionally to the .338 Winchester Mag, and that necked down .338 cases can be safely fired in the .308 Norma chamber due to them both headspacing from the belt. They caution that the resulting case will be "slightly shorter" than the .308 Norma case. Their test rifle was a 24" barreled Springfield custom. High velocities for 150 grain jacketed bullets, using a number of contemporary powders, were over 3,300 fps; the 180 grain loads topped 3,100 fps; 200 grain spitzer Top loads were in the 2,950 range. .300 Winchester Magnum loads in the same manual were fired in a 24" barreled Winchester Model 70; there is a barely noticeable increase over the loads they developed for the .308 Norma. With modern powders and modern pressure measuring equipment, I would expect a bit more of a gap in favor of the Winchester simply because it's boiler is bigger.
